This invention relates to an improved epoxidized polyalkyleneamine-amide resin. The resin of this invention is not only efficient in imparting wet strength to paper, but also paper treated with the resin of this invention can be repulped under substantially less demanding conditions than paper treated with conventional, commercially available epoxidized polyalkyleneamine-amide resin, when papers with the same degree of wet strength are compared.
Paper is sheet material comprising bonded small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper is usually made from cellulose fibers although occasionally, synthetic fibers are used. Paper made from untreated cellulose fibers retains very little of its original strength after it has been soaked in water, i.e., it has very little wet strength. The wet strength of paper is defined as the resistance of the paper to rupture or disintegration when it is wetted with water. In order to increase the wet strength of paper, wet strength resins have been developed for use in the paper industry. These polymeric substances have been used to impart wet strength to different types of paper, for example, paper towels, facial tissue, bag paper and map paper. In some cases the wet strength resins are added to the slurry; in other cases the resin is applied to partially or completely dried paper.
Wet strength resins currently used in the paper making industry include urea-formaldehyde resins, melamine-formaldehyde resins, polymeric amines and epoxidized resins such as polyalkyleneamine-amide resin epoxidized with epichlorohydrin.
Prior art epoxidized polyalkyleneamine-amide resins have met with commercial acceptance by the papermaking industry because they can be used at neutral pH to impart wet strength whereas other resins, such as urea-formaldehyde resins, are used at other than neutral pH. The use of these epoxidized resins instead of urea-formaldehyde resins not only results in less corrosion of the papermaking machinery but also produces a paper product with improved softness. Commercially acceptable epoxidized polyalkyleneamine-amide resins are described in Keim, U.S. Pat. No. 2,926,116, and Chan et al., U.S. Pat. No. 3,887,510. The main disadvantage of this type of resin is the difficulty in the "repulping" of the broke, i.e., breaking down of waste paper to separate the fibers and disperse them in water. This disadvantage has become more significant as the re-use of waste paper has increased during the last few years.
Waste paper must be repulped in order to use it for papermaking. The ease of repulping paper depends on the degree of wet strength imparted to the paper, the type of chemicals used to achieve the wet strength properties, the type of fiber that is used to make the sheets, the degree of mechanical defibering permissible, and the type of repulping equipment available. In practical operation paper mill operators will want to repulp in as short a time as possible with as little investment in mechanical devices as possible and with minimum damage to fibers. A resin which gives good wet strength to paper but which also gives a paper that can be repulped easily is most desirable.
Paper treated with different types of resins are usually repulped under different conditions. Urea-formaldehyde resin-treated paper is easily repulped by adjusting the pH to about 3. However when this resin is used to make paper, the resin must be used under acidic conditions usually with a pH of about 4. The acidic conditions cause corrosion of expensive papermaking machinery.
Paper treated with polyacrylamide-glyoxal type resin is easily repulped since this resin only gives temporary wet strength to paper. Paper treated with this resin maintains its strength upon instant contact with water but loses the wet strength upon further wetting. This type of paper therefore, it not useful for paper bags or for certain types of wipers.
Epoxidized polyalkyleneamine-amide type resin gives a relatively more permanent wet strength to paper, and is suitable for most grades of paper. This property however makes the paper more difficult to repulp. An extensive laboratory study comparing the repulping characteristics of paper treated with urea-formaldehyde and the repulping characteristics of paper treated with epoxidized polyalkyleneamine-amide type resins was reported in an article published by C. Schmalz (TAPPI Journal 44, 275-280, April, 1961). In this laboratory study, handsheets made with different types of brokes such as polyamide-type wet strength brokes, melamine-formaldehyde wet strength brokes, and urea-formaldehyde wet strength brokes were defibered in a TAPPI disintegrator according to the TAPPI standard method T205 M-58. The handsheets were defibered at various levels of pH, with or without addition of hypochlorites and at varous temperatures. Samples were removed at different intervals of time. The samples were used to make handsheets and these handsheets were then compared with standards to ascertain the degree of fiber separation.
The study concluded that in order to defiber paper treated with epoxidized polyalkyleneamine-amide resin in a reasonable period of time, the addition of sodium hydroxide to maintain a pH of at least 10 or higher, a temperature of more than 50.degree. C., and the addition of sodium or calcium hypochlorite was required. It was also concluded that unbleached paper is generally more resistant to defiberation. Unbleached paper requires the use of excessive amounts of hypochlorite because the extra hypochlorite is used up in bleaching the unbleached fiber. The need to use excessive amounts of hypochlorite is not desirable. Hypochlorite changes the optical properties of unbleached paper. An excessive amount of hypochlorite ion in the slurry adversely affects the wet strength efficiency of the resin in paper made from the repulp.